Complementary relationship with a convective boundary layer model to estimate regional evaporation

Abstract

An idea to use a simple convective boundary layer (CBL) model in the complementary relationship to estimate regional evaporation was explored. The CBL model simulated the potential specific humidity deficit D in CBL when the bulk stomatal resistance rst = 0. This value of D was then used in the Penman-type equation to derive evaporation Epo that would occur with ample soil moisture under the prevailing weather condition. The same equation was also used to produce the potential evaporation Ep with the actual humidity deficit, and these Ep and Epo values allowed the evaluation of the actual evaporation E by applying the complementary relationship E = ηEpo - Ep, where η is assumed as 2.0. This was tested with the data set obtained in Hexi Corridor desert area in northwestern China with a modified version of a simple CBL model developed by Lhomme [1997]. It was found that the method produced better estimates of the daytime mean E values with smaller bias than those obtained from a conventional application of the complementary relationship without the CBL model. Also, it was shown that the assumption of η = 2.0 in the complementary relationship was only approximate in most cases. To take this into account, an additional procedure was explored in which η was treated as a variable, and an iteration process with the CBL model determined the final η and E values. It was found that this process produced E values that have smaller systematic error and agree better with the measurements on average, but the unsystematic error got worse than that found with η = 2.0, probably because of use of the CBL model with rst ≠ 0 in the iteration process.